This hook has been derived from the official "encrypt" hook in archlinux. This hook allows to type a single passphrase at startup to decrypt multiple LUKS encrypted devices using dmcrypt. For this to work all devices must have been configured using the same passphrase.
This file needs to be copied to /usr/lib/initcpio/hooks and the "fdencrypt" hook must be listed in the HOOKS variable in /etc/mkinitcpio.conf for it to take effect. The kernel boot command line needs to have "cryptdevice=auto"